The present invention relates to a rapid and accurate photometric means for displaying exposure time by digital or analog displays, wherein the exposure time is determined by an associated density measurement of a projected negative and said exposure time includes reciprocity compensation for the photographic paper, and wherein said density measured and displayed by digital means is representative of a particular print reflectivity. Further photometric means relate to measuring and displaying by digital means, the optical densities of compensating filters required to obtain a color balance of a photographic print from a color negative. An additional photometric means relates to exposure-time measurements by a digital exposure meter wherein the exposure time displayed by digital means is inversely proportional to the light intensity of a projected negative. A drawback in the prior art has been that most devices for this purpose utilize ammeters and the like, which are subject to drift and mechanical offset introducing uncertainty in the readings as well as consuming an operator's time locating the position of a deflected needle relative to a dimly illuminated scale. Furthermore, to provide reasonably accurate measurements and sensitivity, a high-cost delicate meter must be used. Selectable reciprocity effect compensation for a photographic paper cannot be utilized in a meter instrument, and no means are provided in prior art to determine exposure time by density measurements of a negative regardless of the light level.
We have circumvented these drawbacks by developing an economical instrument capable of accurately and rapidly measuring and digitally displaying optical densities of color compensating filters for photographic color printing and exposure times by density measurements, utilizing among other things, memory gain means to vary the signal gain, a signal integrator coupled to the memory gain means and to a voltage comparator, a reset voltage pulse generator controlling a plurality of gates, an exponential decaying voltage source coupled to the voltage comparator, a voltage pulse generator generating a reference voltage pulse of predetermined duration, a gatable clock controlled by the voltage comparator and reference voltage pulse, decimal counter, latch and a digital read-out to display optical density or optical density differences, a binary counter controlling the slope of a second integrator voltage, a second voltage comparator coupled to the second integrator and to a fixed or variable voltage source, a gate coupled to the second voltage comparator controlling the integration interval of the signal integrator and gating a second gatable clock, a decimal counter, latch and digital readout to display exposure time.